Adjust can_be_explored() function of modes/view.c

Rename it to get_file_to_explore().
Add const qualifier to the view parameter.
Use brakets for the buf parameter.
Add get_link_target_abs() function to utils/fs.c

This function resolves a symbolic link to full path of the target.
It will be used in several places to reduce code duplication.
Use get_link_target_abs()

It is used in three places:
 - determining whether a symbolic link is broken
 - checking possibility of previewing file in quickview
 - checking possibility of previewing file in view mode
Improve interface of get_file_to_explore()

Add function comment.
Pass output buffer size as a parameter.
Return zero code on error, instead of returning boolean value.
Merge branch 'fix-relsymlink-exploring'