shared/install: rename 'UnitFileInstallInfo' to 'InstallInfo'

- shorter is better
- name now matches the defining-file name

I was also considering UnitInstallInfo. Can change if people prefer that.

src/shared/install-printf.h

@@ -6,6 +6,6 @@
 
 int install_name_printf(
                 LookupScope scope,
-                const UnitFileInstallInfo *info,
+                const InstallInfo *info,
                 const char *format,
                 char **ret);