library: Use standard major minor macros

library: Use standard major minor macros

I'm not sure why you would roll your own macros for major and minor
while the standard includes have these defined. Using our versions
causes two problems:
 - Some systems don't use this format for their minor/major
 - If the kernel proc interface becomes a 64-bit number, like
   dev_t is in the library, then our macro will need to be changed.

autoconf already had the check and as a bonus for anyone that
puts these definitions in sys/mkdev.h it handles that too.

So this is now the standard way of getting a minor/major number out of a
device id. Examining bits/sysmacros.h showed that their defines are
close to what devname.c had, except it can handle 64-bit numbers.

Signed-off-by: Craig Small <csmall@dropbear.xyz>