Revert "Get rid of ExecStart=agetty lines"

This reverts commit 62bc981.

Reason:
- it is possible to override ExecStart for quite some time in systemd.
- upstream added some useful reasoning how ExecStart arguments were chosen.

units/console-getty.service.m4.in

@@ -20,6 +20,10 @@ Before=getty.target
 ConditionPathExists=/dev/console
 
 [Service]
+# The '-o' option value tells agetty to replace 'login' arguments with an
+# option to preserve environment (-p), followed by '--' for safety, and then
+# the entered username.
+ExecStart=-/sbin/agetty -o '-p -- \\u' --noclear --keep-baud console 115200,38400,9600 $TERM
 Type=idle
 Restart=always
 UtmpIdentifier=cons

units/container-getty@.service.m4.in

@@ -25,6 +25,10 @@ Conflicts=rescue.service
 Before=rescue.service
 
 [Service]
+# The '-o' option value tells agetty to replace 'login' arguments with an
+# option to preserve environment (-p), followed by '--' for safety, and then
+# the entered username.
+ExecStart=-/sbin/agetty -o '-p -- \\u' --noclear --keep-baud pts/%I 115200,38400,9600 $TERM
 Type=idle
 Restart=always
 RestartSec=0

units/getty@.service.m4

@@ -37,6 +37,10 @@ ConditionVirtualization=|lxc-libvirt
 
 [Service]
 # the VT is cleared by TTYVTDisallocate
+# The '-o' option value tells agetty to replace 'login' arguments with an
+# option to preserve environment (-p), followed by '--' for safety, and then
+# the entered username.
+ExecStart=-/sbin/agetty -o '-p -- \\u' --noclear %I $TERM
 Type=idle
 Restart=always
 RestartSec=0

units/serial-getty@.service.m4

@@ -30,6 +30,10 @@ Conflicts=rescue.service
 Before=rescue.service
 
 [Service]
+# The '-o' option value tells agetty to replace 'login' arguments with an
+# option to preserve environment (-p), followed by '--' for safety, and then
+# the entered username.
+ExecStart=-/sbin/agetty -o '-p -- \\u' --keep-baud 115200,38400,9600 %I $TERM
 Type=idle
 Restart=always
 UtmpIdentifier=%I