Add support for number unit suffixes

There are a number of cases in the configuration where one needs to set
a size (think buffer sizes), and counting zeroes is extremely annoying
and unintuitive. For this reason, we introduce number unit suffixes for
kilo-, mega- and giga-: K, M, G, respectively (case-insensitive,
optionally followed by a 'b' or 'B', for byte). The parser also
understands KiB, MiB, etc to indicate base-two units.

Anywhere where you can specify a number, you'll be able to specify one
with a unit suffix from now on:

 log-fifo-size(200M)

A function to parse longs already existed in basic-funcs (tf_parse_int),
which was moved to lib/parse-number.c now, and enhanced to recognise the
unit suffixes too.

This also closes #28.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>