Improve signal handling.

[guymcswain]

I. The pigpio library's default signal handler is changed:

• DMA engines are stopped
• PID lockfile is removed
• The system default handler (SIG_DFL) is re-installed
• The signal is raised again (with Term disposition, see II).

The philosophy here is to do only the essential halting of the DMA engines and avoid any functions that are not signal-safe.

II. The default signal handler is installed only for signals:

• Having SIG_DFL as the current handler. This prevents signals having been ignored or previously installed with custom handlers from getting overridden.
• Having Term disposition as defined in signal(7).

III. SIGPIPE, SIGCHLD and SIGWINCH are ignored as before.

IV. SIGUSR1, SIGUSR2 are custom handlers as before.

V. API gpioSetSignalFunc() can set / cancel custom handlers on any signal except:

• Any signal that was ignored prior to gpioInitialize() or as listed in III.
• Any signal that was preset prior to gpioInitialise().

A cancelled signal will have its previous handler, which may be the library's default handler, re-installed.

VI. Upon gpioTerminate(), any signal installed by the library will be restored to its original handler. (issue #341)