MINOR: guid: restrict guid format

MINOR: guid: restrict guid format

GUID format is unspecified to allow users to choose the naming scheme.
Some restrictions however are added by this patch, mainly to ensure
coherence and memory usage.

The first restriction is on the length of GUID. No more than 127
characters can be used to prevent memory over consumption.

The second restriction is on the character set allowed in GUID. Utility
function invalid_char() is used for this : it allows alphanumeric
values and '-', '_', '.' and ':'.