add slurp_proc_file()

slurp_file() cannot be used to read proc files, as they are returning
a size of zero in stat(). Add a function slurp_proc_file() which is
similar to slurp_file(), but doesn't require the size of the file to
be known.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Simon Horman <horms@verge.net.au>