utils: Greatly improve performance of osmo_hexdump routines

In the osmo-bts and libosmo-abis code the hexdump routine is used
for every incoming/outgoing packet (including voice frames) and the
usage of snprintf showed up inside profiles.

There is a semantic change when more than 4096 characters are used.
The code will now truncate at byte boundaries (and not nibbles).

Code:
 static const int lengths[] = { 23, 1000, 52 };
 char buf[4096];
 int i;

 for (i = 0; i < 30000; ++i)
     char *res = osmo_hexdump(buf, lengths[i & 3]);

Results:

before:					after:
real    0m3.233s			real    0m0.085s
user    0m3.212s			user    0m0.084s
sys     0m0.000s			sys     0m0.000s

0 files changed, 0 insertions, 0 deletions