Simplify error handling of tty_putc

[ghost]

putc returns EOF on errors, not ERR (although both are defined as -1) ...

[mptre]

The ERR -> EOF looks good. tty_putc does not perform any error
handling since it's used with tputs(3), quoting the manual:

... putc is a putchar-like routine to which the characters are passed,
one at a time.

I don't know if it's advised to perform error handling inside putc if
tputs relies on getting EOF back and perform any internal cleanup.

[ghost]

if tputs relies on getting EOF back and perform any internal cleanup.

http://pubs.opengroup.org/onlinepubs/7908799/xcurses/putp.html says:

The tputs() function ignores the return value of putfunc.

quoting man putc here:

fputc(), putc() and putchar() return the character written as an unsigned char cast to an int or EOF on error.

[mptre]

Well then, thanks! Tweaked and committed as 1585928.