0069-proc/readproc.c: Fix the unhex() function.

0069-proc/readproc.c: Fix the unhex() function.

This function is unused (SIGNAL_STRING is defined by default, and if it
is not, procps does not compile -- for example, there is no "outbuf" in
help_pr_sig()) but fix it anyway. There are two bugs:

- it accepts non-hexadecimal characters (anything >= 0x30);

- "(c - (c>0x57) ? 0x57 : 0x30)" is always equal to 0x57.

---------------------------- adapted for newlib branch
. newlib doesn't use that 'unlikely' crap

Signed-off-by: Jim Warner <james.warner@comcast.net>