net-snmp: fix newline when RestrictOID is neither yes nor no

net-snmp: fix newline when RestrictOID is neither yes nor no

snmpd_access_default_add, snmpd_access_HostName_add, and
snmpd_access_HostIP_add check if RestrictOID is `yes` or `no`
but fail to write a newline for other values of RestrictOID
(e.g., `true`, `false`) or if unset, corrupting the generated config.

Fix by using config_get_bool and restructuring to write either
a complete line or nothing. Also require RestrictedOID to be
non-empty when RestrictOID is enabled.

This is a breaking change for configs where RestrictOID is set
to a value that the old code did not match as `yes` (e.g., `true`,
`1`, `on`, `enabled`) but config_get_bool evaluates as true. In
practice, this is unlikely to affect existing installs unless a
user is unknowingly relying on unintended behavior or is deliberately
relying on the old code's non-standard boolean evaluation; this
latter case is considered unlikely by this commit's author. Configs
where RestrictOID is enabled but RestrictedOID is empty are also
affected; previously a directive with a trailing space was written,
now no directive is written.

Signed-off-by: Eric McDonald <redacted>