Use glib functions for base64 decoding/encoding

This fixes several coverity warnings about 'tainted data index sink' and
a fixme about thread safety in the old base64_decode implementation.

Had to adapt the code that used base64_encode_real:

 - oauth.c: different character set order, but it's for the nonce so it
   doesn't matter
 - libyahoo2.c: used as part of the auth, changes "+/=" into "._-".
   Fixed by encoding first the usual way through glib, then replacing.
twitter: Use g_strcasecmp for screen_name

Fixes a few minor bugs with users who have uppercase characters in nicks
(see trac ticket 1138). Most notably, the "follow" command showing no
feedback that a user was followed
Set signal handlers for SIGCHLD and SIGPIPE to SIG_IGN

First part of fixing the potential deadlocks that can happen when
log_message() is called inside sighandler()

The logging messages for these were not printed at all before, anyway,
because the info log level was never attached to anything.

In the case of SIGCHLD this also allows cleaning up zombies with no
extra code
Attach less signals to sighandler, use the default behavior

Nuked: SIGILL, SIGBUS, SIGFPE, SIGQUIT, SIGXCPU.

Left SIGSEGV alone, going to try to do something nice about it.
Separate crash handler for SIGSEGV
Actually remove pointless log_message calls from sighandler
Ceci n'est pas une pipe
rename stuffs
/*...*/
Use irc_write_all() in irc_abort() to ensure it gets sent