libglusterfs: Don't allow '-0' as input value for numbers

Problem:
io-stats tries to init log-buf-size as uint32. All gf_string2u*** functions
which get the unsigned values from string don't want the string to contain '-'.
So the GF_OPTION_INIT with '-0' as value fails init in io-stats, but by that
time 'ret' is already reset to 0. Io-stats ends up returning 0 even when init
failed. Because of this caller of init thinks initialization is successful when
it is not. iostat_xlator->private is still NULL.  Because of this when a fop
tries to access members of io-stat-private structure, it crashes.

Fix:
I initially thought may be we should fix all gf_string2u*** functions to
accept '-0'. But all these functions are used only for setting volume options.
If we accept '-0', gluster volume info shows output as follows:

diagnostics.brick-log-buf-size: -0

This seemed ugly, so I felt it is better to disallow '-0' as valid input for
numbers.

Also fixed return value in cases of failures in io-stats.

Change-Id: I67ac92853b6d2be70516ad1d07505ffd9f058aa4
BUG: 1111557
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8129
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>