Reduce scope of some variables in update_info_file

Suggested by cppcheck.
Use scanf with width limit read_optional_number

Should prevent possible crash on huge number in the input.

Suggested by cppcheck.
Don't assign variable to itself

In filelist.c:clear_current_line_bar()

Suggested by cppcheck.
Fix file descriptor leak in io/iop:iop_cp()

Suggested by cppcheck.
Remove second chosp() on the same path in a row
Add a comment in fileops.c:put_next()
Merge branch 'cppcheck-code-fixes'

Fix small issues discovered by cppcheck.
Add constant of valid platform path separators
Allow relative paths in executable()
Merge branch 'handle-relative-paths-in-executable-func'

Allow relative paths for executable() builtin function.
Skip estimations for slowfs sources
Skip estimations if system calls are not used
Do not count directories in estimations

They are quite misleading in progress as it's files which are
time consuming to copy.
Fix ops:ops_alloc() function comment
Better version of 240ae2e (skip eta on nosyscalls)
Add base_dir field to ops_t structure
Display relative paths in IO messages
Extract ui:ui_sb_quick_msgf() function

Immediately (UI is updated) displays message on the status bar without
storing it in message history.
Merge branch 'minor-io-updates'

Several functional/code enhancements related to ETA of IO.
Remove odd excess check in engine/cmds