swtpm_setup: add abstract swtpm_backend_ops with dir:// implementation

swtpm_setup: add abstract swtpm_backend_ops with dir:// implementation

Abstract away implementation specific code for handling TPM state in
swtpm_setup. The current code for handling directories is moved to
'swtpm_backend_dir.c'.

Where possible, the input argument is simply passed verbatim as
'backend-uri' to swtpm.

No functional change intended, aside from supporting 'dir://' as
optional prefix. The checks for lock-file accessibility are moved to
check_access(), but that shouldn't affect anything AFAICT.

Signed-off-by: Stefan Reiter <stefan@pimaker.at>