src/server/conf.c: Don't die in parse_include()

Let the callers of the parse_include() function check for errors.  This
is just a cosmetic change, the behaviour remains the same.
Teach the client about backslash escape sequences

Let send_nsca(8)'s "-d" and "-e" flags accept C-style escape sequences,
so that e.g. "-e '\n'" can be specified.  Octal numbers with a leading
zero and hexadecimal numbers prefixed with "0x" are also accepted.