Avoid undeclared identifiers when compiling with older kernel headers.

So we get around the PATH_MAX limitation. Also a tiny bit more
efficient, I hope.

Silly one-character typo that causes directory apparent sizes to be very
off in some scenarios.

Reported & patched by Andrew Neitsch.

Fixes #150

(cherry picked from commit a076ac714aa6fb24e0e9eb227372df84a4b6000b)

(cherry picked from commit ef7b4e5c28739c26b3a120296f9dc2ddaf2d4375)

(cherry picked from commit 9f28920a64c155957b83324cd3a3cf0fda6020c9)

(cherry picked from commit ad5b7fce74a3b7b0ed726620c81ea552c643cdad)

(cherry picked from commit 2faefc3b2432409f3ccca0b6ab53a228d5b9b00d)

(cherry picked from commit 32b77d0064d3e5f52e9204ddeffc29bb21a67753)

(cherry picked from commit ce7036d249b6f05633c343ab95d88552aab85402)

(cherry picked from commit 9801f46ece0ca2525f02d71464efc42296dddcb5)

(cherry picked from commit e4e8ebd9e0037812436a1588809deb23e0f3751a)

Fixes #141

Reduces memory by a tiny bit. Arguably we never needed tombstones
because entries are never removed, so there shouldn't be any performance
hit there. We don't even need a 'used' flag either, considering that can
be represented by a NULL value, but I'm not really up for
implementing/modifying my own hash table.

ref: https://attractivechaos.wordpress.com/2019/12/28/deletion-from-hash-tables-without-tombstones/

They were defined (rather than just declared) in .h files, when they
really should be defined in at most a single .c file.

Fixes #140.

Fixes #135

Broken in 77aca35f
Fixes #134

This is a best-effort approach to save ncdu state when memory is low.
There's likely allocation in libraries that isn't being checked
(ncurses, printf).

Fixes #132 (it actually doesn't, that needs a 64bit static binary too,
but I'll get to that)

Fixes #128.

This allocation is currently leaked, but as long as we don't allocate
new ones for each refresh, that shouldn't be much of an issue.

(cherry picked from commit 9dc2d32a8fb557112f8f96cd14d50b934543ce39)

stat_to_dir() assumes that buf_dir is clean; calling it twice breaks
that asumption.

Symlink loops are handled by the stat(2) syscall. Symlinks pointing to a
directory are ignored (to avoid loops in the recursive scan).

This adds an 'm' command to show the latest modified time of all files
in a directory. The 'M' command allows for ascending and descending
mtime sorting. These are only enabled with the -e flag and overload
the dir_ext mtime field.

I had taken care to not sort empty directories during dirlist_open(),
but forgot that manual user actions can still cause dirlist_set_sort()
to be called, which does not handle empty directories.

Reported by Alex Wilson.

https://dev.yorhel.nl/ncdu/bug/125

Based on https://dev.yorhel.nl/ncdu/bug/124

Implements https://dev.yorhel.nl/ncdu/bug/43