"signal" is a C library function, not a variable
Rename "dont_fork" to "daemon_mode"

Positive statements are easier to reason about.  This is demonstrated
by the redundant "if (dont_fork == false)" checks, and the check
for foreground_mode which actually checks for daemon mode.
Move STDIN / STDOUT mangling to radlog_init()

Which means fewer global functions that have only one caller,
and all of the STDIN / STDOUT mangling is now encapsulated in
log.c