Add fid_to_string and string_to_fid functions for more readable print/scan statements. Allow for error checking, defaults, etc.

Example print statement:

Original: C_DEBUG(D_PAGE, "%lu@"DFID" %p %lx %d\n",
idx, PFID(&hdr->coh_lu.loh_fid), vmpage, vmpage->private, type);

New: C_DEBUG(D_PAGE, "%lu@%s %p %lx %d\n",
idx, fid_to_string(hdr->coh_lu.loh_fid), vmpage, vmpage->private, type);

Example scan:

Original: if (sscanf(opt.o_src, SFID, RFID(&old_fid)) != 3 || ...

New:
old_fid = string_to_fid(opt.o_src);
if ( ...