varlink: add enum types for configuration settings in io.systemd.Manager

varlink: add enum types for configuration settings in io.systemd.Manager

Convert 8 string fields in the io.systemd.Manager varlink interface to
proper enum types:

- LogTarget: new enum (console, console_prefixed, kmsg, journal, ...)
- DefaultStandardOutput/Error: reuse ExecOutputType from common
- DefaultMemory/CPU/IOPressureWatch: reuse CGroupPressureWatch from common
- DefaultOOMPolicy: new enum (continue, stop, kill)
- CtrlAltDelBurstAction: reuse EmergencyAction from common

Output serialization updated to use JSON_BUILD_PAIR_ENUM for automatic
underscorification of dash-containing values.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>