Allow rewriting of NIT table

[hack03er]

I would like to propose this feature to MuMuDVB. In my use case this enables me
to reduce a stream to only a few frequencies, which we forward to our CMTS.
Several MuMuDVB instances are streaming a frequency, which allows the CMTS to
establish a reduced stream again. The patching of the frequencies in the NIT is
only implemented for cable_delivery_system_descriptor.
An exemplary config for the first instance would be:

# --------------- TUNING ----------------
delivery_system=DVBC_ANNEX_AC # DVBC_ANNEX_B
card=0
freq=330000
srate=6900

# ---------- AUTOCONFIGURATION ----------
autoconfiguration=full
autoconf_radios=1

# -------------- NETWORKING -------------
autoconf_ip4=239.192.%card.%number
common_port=10000
multicast_ttl=16
multicast_ipv4=1
multicast_ipv6=0
rtp_header=1

# ------------ SAP ANNOUNCES ------------
# MuMuDVB patched to not include channels without name
sap_default_group=%type
sap_organisation=nw-cable

# ---------- WHOLE TRANSPONDER ----------
new_channel
ip=239.255.0.1
port=10000
pids=8192

# ----------- TABLE REWRITING -----------
# For full autoconf the pat is rewritten which breaks the full transponder
# streaming. As we stream all PIDs for each SID no rewrite is required.
rewrite_pat=0

# send EPG Data only for the current SID (or all for whole transponder)
#rewrite_eit=1
rewrite_eit=0

rewrite_nit=1
# E58-E61 band
freq_patch=0x03300000->0x07700000
freq_patch=0x04500000->0x07780000
freq_patch=0x04660000->0x07860000 # TSID 10000
freq_patch=0x05620000->0x07940000 # TSID 10017

The feature implements NIT-rewriting in the way that only given frequencies are kept.
If you agree, I would also replace void setCRC32(unsigned char *data) and
bool table_needs_update(char *mod_log_module, const int stored_version, unsigned char *buf)
in the other modules.

I tested the check for a new nit version with this:

diff --git a/src/rewrite_nit.c b/src/rewrite_nit.c
index 58b3c62..629e7d6 100644
--- a/src/rewrite_nit.c
+++ b/src/rewrite_nit.c
@@ -357,6 +357,7 @@ void get_nit_packet(const nit_section_t *nit_section_array, int section_number, 
 
 int global_packet_number = -1;
 int global_section_number = -1;
+int nit_section_count = 0;
 
 /**
  * @brief Swapping the NIT packet with a patched packet for every channel
@@ -374,7 +375,16 @@ void nit_rewrite_new_global_packet(unsigned char *ts_packet, rewrite_parameters_
 			log_message(log_module, MSG_DEBUG, "NIT not yet valid, we get a new one (current_next_indicator == 0)");
 			return;
 		}
+		nit_section_count++;
+		if (nit_section_count % 50 == 0) {
+			log_message(log_module, MSG_DEBUG, "NEW!");
+		}
+		nit_t *nit_header = (nit_t *)(ts_packet + NIT_TS_HEADER_LEN + 1);
+		nit_header->version_number += nit_section_count / 50;
+		nit->version_number += nit_section_count / 50;
+
 		if (table_needs_update(log_module, rewrite_vars->nit_version, ts_packet)) {
+			log_message(log_module, MSG_INFO, "New NIT version. Start caching NIT sections.");
 			rewrite_vars->nit_version = nit->version_number;
 
 			rewrite_vars->nit_section_count = nit->last_section_number + 1;