env: common: accept flags on reset to default env

The function set_default_env() sets the hashtable flags for import_r().
Formally set_default_env() doesn't accept flags from its callers. In
practice the caller can (un)set the H_INTERACTIVE flag, but it has to be
done using the first character of the function's string argument. Other
flags like H_FORCE can't be set by the caller.

Change the function to accept flags argument. The benefits are:
1. The caller will have to explicitly set the H_INTERACTIVE flag,
   instead of un-setting it using a special char in a string.
2. Add the ability to propagate flags from the caller to himport(),
   especially the H_FORCE flag from do_env_default() in nvedit.c that
   currently gets ignored for "env default -a -f" commands.
3. Flags and messages will not be coupled together. A caller will be
   able to set flags without passing a string and vice versa.

Please note:
The propagation of H_FORCE from do_env_default() does not introduce any
functional changes, because currently himport_r() is set to destroy the
old environment regardless if H_FORCE flag is set or not. More changes
are needed to utilize the propagation of H_FORCE.

Signed-off-by: Yaniv Levinsky <yaniv.levinsky@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
diff --git a/include/environment.h b/include/environment.h
index 2fe1f3e..5e90f15 100644
--- a/include/environment.h
+++ b/include/environment.h
@@ -272,7 +272,7 @@
 char *env_get_default(const char *name);
 
 /* [re]set to the default environment */
-void set_default_env(const char *s);
+void set_default_env(const char *s, int flags);
 
 /* [re]set individual variables to their value in the default environment */
 int set_default_vars(int nvars, char * const vars[], int flags);