server: Improve signal handling under Linux

server: Improve signal handling under Linux

Commit 5087a955 added custom signal handlers for the openocd
server process.

Before this commit, when openocd is run as a background process
having the same controlling terminal as gdb, Control-C would be
handled by gdb to stop target execution and return to the gdb prompt.

However, after commit 5087a955, the SIGINT caused by pressing
Control-C also terminates openocd, effectively crashing the
debugging session.  The only way to avoid this is run openocd in
a different controling terminal or to detach openocd from its
controlling terminal,
thus losing all job control for the openocd process.

This patch improves the server's handling of POSIX signals:
1) Keyboard generated signals (INT and QUIT) are ignored
   when server process has is no controlling terminal.
2) SIGHUP and SIGPIPE are handled to ensure that .quit functions
   for each interface are called if user's logs out of X
   session or there is a network failure.

SIG_INT & SIG_QUIT still stop openocd
when it is running in the foreground.

Change-Id: I03ad645e62408fdaf4edc49a3550b89b287eda10
Signed-off-by: Brent Roman <genosensor@gmail.com>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/3963
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>