trailers: export action enums and corresponding lookup functions

Separate the mechanical changes out of the next patch.  The functions
are changed to take a pointer to enum, because struct conf_info is not
going to be public.

Set the default values explicitly in default_conf_info, since they are
not anymore close to default_conf_info and it's not obvious which
constant has value 0.  With the next patches, in fact, the values will
not be zero anymore!

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trailer.h

@@ -1,6 +1,28 @@
 #ifndef TRAILER_H
 #define TRAILER_H
 
+enum trailer_where {
+	WHERE_END,
+	WHERE_AFTER,
+	WHERE_BEFORE,
+	WHERE_START
+};
+enum trailer_if_exists {
+	EXISTS_ADD_IF_DIFFERENT_NEIGHBOR,
+	EXISTS_ADD_IF_DIFFERENT,
+	EXISTS_ADD,
+	EXISTS_REPLACE,
+	EXISTS_DO_NOTHING
+};
+enum trailer_if_missing {
+	MISSING_ADD,
+	MISSING_DO_NOTHING
+};
+
+int trailer_set_where(enum trailer_where *item, const char *value);
+int trailer_set_if_exists(enum trailer_if_exists *item, const char *value);
+int trailer_set_if_missing(enum trailer_if_missing *item, const char *value);
+
 struct trailer_info {
 	/*
 	 * True if there is a blank line before the location pointed to by