Explicitly cast return value of read(2) to "int"

Explicitly cast return value of read(2) to "int"

This fixes the following gcc warning, emitted by Xcode:

src/ngircd/sighandlers.c: In function 'Signal_Callback':
src/ngircd/sighandlers.c:239: warning: implicit conversion shortens 64-bit value into a 32-bit value