the swap entry loop read the first entry repeatedly instead of
iterating with sep[i]. also free the allocation on error paths
and fix "SWAP_STATA" typo in the warning message.

Check for `< 0` instead of `== -1`.

Fixes coding style. Formatting commits suck, incoherent coding style
sucks more.
https://suckless.org/coding_style/

Fixes coding style. Formatting commits suck, incoherent coding style
sucks more.
https://suckless.org/coding_style/

Fixes coding style. Formatting commits suck, incoherent coding style
sucks more.
https://suckless.org/coding_style/

Except stated otherwise in comment.

Fixes coding style. Formatting commits suck, incoherent coding style
sucks more.
https://suckless.org/coding_style/

function prototype with unspecified argument is obsolete since c99.

additionally some of these function which don't take any argument were
being called with a `const char *` arg, which is UB.

fix both these issues by declararing ALL the components to accept a
`const char *`, and name the arg "unused" if it's meant to be ignored.

this gives the compiler a chance to check weather the prototype and
definiton matches or not, which would catch issues like 3c47701.

 - Remove <errno.h> because related functions are in util.c now
 - Remove corresponding <string.h> if not used otherwise