pfexec become plugin: fix broken defaults for illumos/SmartOS

The pfexec become plugin has had incorrect defaults since it was
migrated from Ansible core, making it unusable on illumos without
manual workarounds:

1. become_flags defaulted to '-H -S -n' which are sudo flags.
   pfexec does not accept any of these options, causing:
   'exec: illegal option -- H'

2. wrap_exe defaulted to false. Unlike sudo, pfexec does not
   interpret shell constructs internally. Since Ansible generates
   compound commands (echo BECOME-SUCCESS-xxx ; python3), these
   must be wrapped in /bin/sh -c for pfexec to execute them.

These issues were originally reported in 2016 (ansible/ansible#15642),
migrated to community.general as #3671, and partially fixed by PR #3889
in 2022 (which corrected quoting but not the defaults). Users have had
to work around this with explicit inventory settings ever since.

Changes:
- become_flags default: '-H -S -n' -> '' (empty)
- wrap_exe default: false -> true
- build_become_command: handle empty flags cleanly
- Updated tests to match corrected defaults
- Added test for custom flags
- Improved wrap_exe description to explain why it should be enabled

plugins/become/pfexec.py

@@ -46,7 +46,7 @@ options:
   become_flags:
     description: Options to pass to C(pfexec).
     type: string
-    default: -H -S -n
+    default: ""
     ini:
       - section: privilege_escalation
         key: become_flags
@@ -73,8 +73,12 @@ options:
       - section: pfexec_become_plugin
         key: password
   wrap_exe:
-    description: Toggle to wrap the command C(pfexec) calls in C(shell -c) or not.
-    default: false
+    description:
+      - Toggle to wrap the command C(pfexec) calls in C(shell -c) or not.
+      - Unlike C(sudo), C(pfexec) does not interpret shell constructs internally,
+        so commands containing shell operators must be wrapped in a shell invocation.
+      - This should generally be left enabled.
+    default: true
     type: bool
     ini:
       - section: pfexec_become_plugin
@@ -103,4 +107,7 @@ class BecomeModule(BecomeBase):
 
         flags = self.get_option("become_flags")
         noexe = not self.get_option("wrap_exe")
-        return f"{exe} {flags} {self._build_success_command(cmd, shell, noexe=noexe)}"
+        become_cmd = self._build_success_command(cmd, shell, noexe=noexe)
+        if flags:
+            return f"{exe} {flags} {become_cmd}"
+        return f"{exe} {become_cmd}"