Optimize display_signed_time() - remove g_snprintf

When tshark is printing columns (default) display_signed_time() is called for every frame.
Current implemention is using g_snprintf() which makes this function costly when there is lot frames.

Change-Id: I109c8699d38bfbd05475d457ae4173e937c6812d
Reviewed-on: https://code.wireshark.org/review/1447
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Document to_str-int.h functions.

Change argument name of hex_to_str_back pad->len it seems to sounds better.
Make uint_to_str_back_len() value uint32, to be sure about required buffer size.

Change-Id: I48fd560683c5c0845cbb60813887a18328ec01e6
Reviewed-on: https://code.wireshark.org/review/1448
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>