Fix the manual for old texinfo

Installing the manual fails with texinfo 4.13:

/S/manual/maint.texi:288: Unknown command `lbracechar'.
/S/manual/maint.texi:288: Misplaced {.
/S/manual/maint.texi:288: Misplaced }.
/S/manual/maint.texi:290: Unknown command `rbracechar'.
/S/manual/maint.texi:290: Misplaced {.
/S/manual/maint.texi:290: Misplaced }.
/S/manual/maint.texi:302: Unknown command `lbracechar'.
/S/manual/maint.texi:302: Misplaced {.
/S/manual/maint.texi:302: Misplaced }.
/S/manual/maint.texi:305: Unknown command `rbracechar'.
/S/manual/maint.texi:305: Misplaced {.
/S/manual/maint.texi:305: Misplaced }.
makeinfo: Removing output file `/B/manual/libc.info' due to errors; use --force to preserve.
make[2]: *** [Makefile:142: /B/manual/libc.info] Error 1

	* manual/maint.texi: Use @{ and @}.