Make frag fr_fix unsigned

Make frag fr_fix unsigned

The field only stores unsigned values, so let's make it unsigned to
stop people worrying about the possibility of negative values.

	* frags.h (struct frag <fr_fix>): Use unsigned type.
	* frags.c (frag_new): Assert that current size exceeds
	old_frags_var_max_size.
	* ehopt.c (get_cie_info): Adjust for unsigned fr_fix.
	* listing.c (calc_hex): Likewise.
	* write.c (cvt_frag_to_fill, write_relocs): Likewise.
	* config/tc-arc.c (md_convert_frag): Likewise.
	* config/tc-avr.c (avr_patch_gccisr_frag): Likewise.
	* config/tc-mips.c (md_convert_frag): Likewise.
	* config/tc-rl78.c (md_convert_frag): Likewise.
	* config/tc-rx.c (md_convert_frag): Likewise.
	* config/tc-sparc.c (md_apply_fix): Likewise.
	* config/tc-xtensa.c (next_instrs_are_b_retw): Likewise.
	(unrelaxed_frag_min_insn_count, unrelaxed_frag_has_b_j): Likewise.