Vendored libslirp (#2045)

Add vendored libslirp into the repo with a shim to remove its dependency on glib.

src/frontend/libslirp/.clang-format

@@ -0,0 +1,58 @@
+# https://clang.llvm.org/docs/ClangFormat.html
+# https://clang.llvm.org/docs/ClangFormatStyleOptions.html
+---
+Language:        Cpp
+AlignAfterOpenBracket: Align
+AlignConsecutiveAssignments: false # although we like it, it creates churn
+AlignConsecutiveDeclarations: false
+AlignEscapedNewlinesLeft: true
+AlignOperands:   true
+AlignTrailingComments: false # churn
+AllowAllParametersOfDeclarationOnNextLine: true
+AllowShortBlocksOnASingleLine: false
+AllowShortCaseLabelsOnASingleLine: false
+AllowShortFunctionsOnASingleLine: None
+AllowShortIfStatementsOnASingleLine: false
+AllowShortLoopsOnASingleLine: false
+AlwaysBreakAfterReturnType: None # AlwaysBreakAfterDefinitionReturnType is taken into account
+AlwaysBreakBeforeMultilineStrings: false
+BinPackArguments: true
+BinPackParameters: true
+BraceWrapping:
+  AfterControlStatement: false
+  AfterEnum:       false
+  AfterFunction:   true
+  AfterStruct:     false
+  AfterUnion:      false
+  BeforeElse:      false
+  IndentBraces:    false
+BreakBeforeBinaryOperators: None
+BreakBeforeBraces: Custom
+BreakBeforeTernaryOperators: false
+BreakStringLiterals: true
+ColumnLimit:     80
+ContinuationIndentWidth: 4
+Cpp11BracedListStyle: false
+DerivePointerAlignment: false
+DisableFormat:   false
+IndentCaseLabels: false
+IndentWidth:     4
+IndentWrappedFunctionNames: false
+KeepEmptyLinesAtTheStartOfBlocks: false
+MacroBlockBegin: '.*_BEGIN$' # only PREC_BEGIN ?
+MacroBlockEnd:   '.*_END$'
+MaxEmptyLinesToKeep: 2
+PointerAlignment: Right
+ReflowComments:  true
+SortIncludes:    false
+SpaceAfterCStyleCast: false
+SpaceBeforeAssignmentOperators: true
+SpaceBeforeParens: ControlStatements
+SpaceInEmptyParentheses: false
+SpacesBeforeTrailingComments: 1
+SpacesInContainerLiterals: true
+SpacesInParentheses: false
+SpacesInSquareBrackets: false
+Standard:        Auto
+UseTab:          Never
+...