fish: Allow suffixes on number parameters (eg. 1M)

This small change uses the gnulib xstrtoll functionality to
enable suffixes on integer parameters in guestfish.  For example:

 truncate-size /file 1G

(previously you would have had to given the full number).

This also applies to the 'alloc' and 'sparse' commands (and
indirectly to the -N option).  The specification for these commands
has changed slightly, in that 'alloc foo 1MB' would now use SI
units, allocating 1000000 bytes instead of a true megabyte.  All
existing uses would use 'alloc foo 1M' which still allocates true
megabytes.