Allow setting of variables of type dev_t by indicating the name of

Allow setting of variables of type dev_t by indicating the name of
a special file on the command line, eg:
  sysctl kern.dumpdev=/dev/ad1s1b

In parse(), when a value is given for a CTLTYPE_QUAD variable,
newval and newsize erroneously fail to be set because of an early
"break".

show_var() contains code that duplicates the functionality of the
oidfmt() function.

PR:		33151, 33150
Submitted by:	Thomas Quinot <thomas@cuivre.fr.eu.org>