pmb: Make RunOutputTypeDefault and RunOutputTypePopen enums

This allows us to get rid of some of the validation in sanity_checks()
as mypy handles this validation at "build time", and any typos in the
enum instantiation would be a runtime error rather than a silent
failure.

Additionally, it allows us to encode some of the behaviour of the
different output types into the type definition itself by using methods.

Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2642
(cherry picked from commit 7d2f055bcb)

pmb: Fix PmbArgs containing a string for output

While the annotations were changed to suggest that the output property
of PmbArgs contains a RunOutputType, at runtime it actually contained a
string because the argument parsing code hadn't been adapted to create a
RunOutputType. Fix this, and also change it to RunOutputTypeDefault as
while at it as that's more accurate.

Fixes 7d2f055bcb
Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2644

pmb/helpers/frontend.py

@@ -19,7 +19,7 @@ import pmb.chroot.other
 import pmb.ci
 import pmb.config
 from pmb.core import Config
-from pmb.types import Env, PmbArgs
+from pmb.types import Env, PmbArgs, RunOutputTypeDefault
 import pmb.export
 import pmb.flasher
 import pmb.helpers.aportupgrade
@@ -516,7 +516,7 @@ def stats(args: PmbArgs) -> None:
     # Install ccache and display stats
     pmb.chroot.apk.install(["ccache"], chroot)
     logging.info(f"({chroot}) % ccache -s")
-    pmb.chroot.user(["ccache", "-s"], chroot, output="stdout")
+    pmb.chroot.user(["ccache", "-s"], chroot, output=RunOutputTypeDefault.STDOUT)
 
 
 def work_migrate(args: PmbArgs) -> None: