[pgfkeys] Obsolete \pgfkeys@ifcsname and use \ifcsname?

[muzimuzhi]

When reading pgfkeys.code.tex, I find the use of \pgfkeys@ifcsname. Considering

1. There is already a safe guard in pgfkeys.code.tex which requires etex v2.
2. There is one direct use of \ifcsname in pgfkeys.code.tex.
3. Another etex primitive \scantokens is directly used (see 187f5f4) and its wrapped name \pgfutil@scantokens is made obsolete.

pgf/tex/generic/pgf/utilities/pgfkeys.code.tex

Lines 30 to 46 in c8d7d9b

	\ifx\PackageError\undefined
	\def\pgfkeys@error#1{\errmessage{Package pgfkeys Error: #1}}%
	\else
	\def\pgfkeys@error#1{\PackageError{pgfkeys}{#1}{}}%
	\fi
	\ifx\eTeXrevision\undefined%
	\pgfkeys@error{PGF requires etex in extended mode}%
	\csname @@end\expandafter\endcsname\expandafter\end%
	\fi
	\ifnum\eTeXversion<2
	\pgfkeys@error{PGF requires etex version 2}%
	\csname @@end\expandafter\endcsname\expandafter\end%
	\fi
	\expandafter\let\expandafter\pgfkeys@ifcsname\csname ifcsname\endcsname%

Can we obsolete \pgfkeys@ifcsname and use \ifcsname directly and consistently? (I should be confident enough to just open a PR. hmmm)

[hmenke]

Using \ifcsname everywhere should be fine (probably just forgot that when transitioning to eTeX), \scantokens I'd like to avoid whenever possible, because it has too many gotchas.

[muzimuzhi]

Thanks.

[hmenke]

Feel free to suggest more eTeX optimizations where you see fit. You can always open a PR. If you check the backlog, you'll see that most of them got accepted.