Add validators for iec_size, fq_codel_size, int, job_mode, reboot_parameter, unit_mounts_for

[SJrX]

Goal

Batch 4 of 5.

Targets

Function	Usages	C source	Approach
config_parse_iec_size(0)	4	shared/conf-parser.c:974	parse_size(rvalue, 1024, ...) — IEC byte, same shape as tbf_size
config_parse_fq_codel_size(QDISC_KIND_FQ_CODEL)	4	network/tc/fq-codel.c:267	parse_size(rvalue, 1024, ...) — same IEC byte shape
config_parse_int(0)	4	shared/conf-parser.c:959 (DEFINE_PARSER(int, ..., safe_atoi))	signed int (32-bit range)
config_parse_job_mode(0)	4	load-fragment.c:144 (DEFINE_CONFIG_PARSE_ENUM(... job_mode ...))	Enum from job_mode_table
config_parse_reboot_parameter(0)	4	load-fragment.c:353 → reboot_parameter_is_valid	ASCII printable, length ≤ 255, specifier-aware
config_parse_unit_mounts_for(0)	4	load-fragment.c:3111	Whitespace-separated absolute paths (specifier-aware)

Skipped

• config_parse_string(0) (4 uses) — C parser accepts any string with ltype=0 (no STRING_SAFE/STRING_ASCII flags); a validator here would be functionally equivalent to NullOptionValue and just game the burn-down without helping users.
• config_parse_hw_addrs(0) (4 uses) — multi-format MAC parser (xx:yy:zz:aa:bb:cc, xxyy.zzaa.bbcc, xx-yy-zz-aa-bb-cc, IPv4, IPv6, infiniband). Doable but the regex would be sprawling; defer to batch 5 if useful.
• config_parse_socket_listen(SOCKET_SOCKET) (4 uses) — abstract / IPv4 / IPv6 / Unix path / FD variants, defer.
• config_parse_dhcp_option_tlv(0) (4 uses) — DHCP option TLV byte syntax, defer.
• config_parse_unit_condition_string(CONDITION_VERSION) (8 uses) — operator version or named-version comparisons, defer (would need specialized grammar per condition variant).