`seqtk sample` Long-Form Arguments

[GilChrist19]

Lets start with - I found this issue because I was stupid.

Many cmdline tools accept short and long form arguments.
So, when I read the seqtk sample help, I assumed we could provide either -sXXX or -seed=XXX for the seed:

Usage:   seqtk sample [-2] [-s seed=11] <in.fa> <frac>|<number>

Options: -s INT       RNG seed [11]
         -2           2-pass mode: twice as slow but with much reduced memory

When providing -seed=XXX, seqtk sample doesn't fail, it silently uses 0 as the seed.

This happens because:
The number of arguments provided passes the getopt() call here.
The string check passes here.
Then, atol is passed a null string and initializes kr_srand() with a seed of 0.
(I was expecting the default value, 11, but it behaves like -s0 instead)

The same thing happens if we pass -se as the seed.
(Tested b/c I thought it was parsing e as its ascii value, 101. This is not the case.)
-seed XXX does fail, which is nice.

While it would be cool to parse long-form args, I don't think that's worth the effort.

I think clarifying the documentation is more straightforward:
Change this line from [-s seed=11] to just [-s]
Change this line from RNG seed [11]\n to RNG seed [default=11]\n
That would make the accepted argument clear while also denoting the default seed.

~Jared