configure: --asan=1 parameter for AddressSanitizer

Requires gcc >=4.8 or clang >=3.1

AddressSanitizer (ASan) is a fast memory error detector. See also:

https://code.google.com/p/address-sanitizer/wiki/AddressSanitizer
Revert "Twitter search channels" to sync with master

This reverts commit f370af0a95ab803486705d32df859fc947140a12.

It never landed in master because it 'leaks' contacts, as in they never
get removed from the channel/blist until the account gets removed, which
becomes a big issue with high activity hashtags.

Sending it back to review, will apply again once it's fixed.
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